1. Technical Field
The present invention relates to a welding method for welded members subjected to fluoride passivation treatment, to a fluoride passivation retreatment method, and to welded products.
2. Background Art
In, for example, semiconductor manufacturing lines, because a fluorine system gas is supplied in a stable manner over a long period of time in a fluorine system gas supply line (chiefly, in fluorine system gas supply lines for excimer laser steppers), a fluoride passivated film is formed on the inner surfaces of the piping parts which comprise the fluorine gas supply system.
When members to be welded (for example, piping, valves, and the like) which are subjected to fluoride passivation treatment are welded together to produce a fluorine gas supply line, it has been discovered that the gas which is supplied from this fluorine gas supply line becomes contaminated with particles and dust (FIG. 3).
When members to be welded which have fluoride passivated films formed thereon are welded, the fluoride passivated film disappears in the welded part.
For this reason, there are occasions on which the durability with respect to fluorine system gases (chiefly, fluorine and hydrogen fluoride) worsens.
Thus, the reformation of a fluoride passivated film is conducted.
However, when welding has been conducted, even if a fluoride passivated film is again formed, the gas which is supplied from the fluorine gas supply line constructed using such welded members is contaminated by particles and dust. Furthermore, it is not always the case that the decline in durability with respect to fluorine system gases can be overcome.
The present invention has as an object thereof to provide a welding method for members to be welded which are subjected to fluoride passivation treatment, and a fluoride passivation retreatment method, which involve no generation of particles or dust, and which provide superior resistance with respect to fluorine system gases, when fluoride passivation retreatment is conducted after welding.
In the welding method for members lo be welded which are subjected to fluoride passivation treatment in accordance with the present invention, when the members to be welded, which comprise stainless steel subjected to fluoride passivation treatment, are welded, hydrogen is added to the gas (the back shield gas) which flows through the members to be welded.
In the welding method for members to be welded which are subjected to fluoride passivation treatment in accordance with the present invention, the thickness of the fluoride passivated film in a predetermined range from the butt end surfaces of the members to be welded, which comprise stainless steel subjected to fluoride passivation treatment, is set to 10 nm or less, and welding is conducted.
Furthermore, in the fluoride passivation retreatment method in accordance with the present invention, after conducting the welding method described above, at least the welded portion is heated, and a gas containing fluorine gas is caused to flow in the interior.
The present inventors have assiduously searched for the cause of the generation of particles and dust even when fluoride passivation retreatment is conducted, and for the reason that the decline in resistance to fluorine system gases cannot be overcome.
The following points have been learned. These are that: (1) the inner surfaces of the members to be welded become rough in the vicinity of the welded part (FIG. 1); (2) crystals of fluorides of iron and chromium are deposited (FIG. 2); and (3) the fluoride passivated film melts during welding, fumes are generated-from-this fluoride passivated-film, and the fumes themselves become particles and dust and are deposited on the inner surfaces of the welded members.
It is assumed that (1), (2), and (3) above are sources of contamination of the gas which flows through the fluorine gas supply lines constructed by welding.
First, the present inventors assiduously looked for a welding method which made it possible to prevent roughness of the inner surface, and which did not involve the generation of deposits.
As a result, it was discovered that, during welding, by adding hydrogen to the gas (the back shield gas) flowing in the members to be welded, it was possible to prevent the occurrence of roughness on the inner surface, and to prevent the generation of deposits, and it was also possible to prevent the generation of fumes and to prevent their deposition onto the inner surface of the welded members; the present invention was arrived at on the basis of this discovery.
Furthermore, it was discovered that if, prior to conducting welding, the thickness of the fluoride passivated film within a range of at least 5 mm from the butt end surfaces of members to be welded comprising stainless steel subjected to fluoride passivation treatment was set to a level of 10 nm or less, it was possible to prevent the occurrence of roughness of the inner surface, and to prevent the generation of deposits.
Additionally, it was confirmed that if fluoride passivation retreatment was conducted after conducting such welding, there was no mixture of particles or dust into the gas, and furthermore, it was possible to restore resistance to fluorine system gasses.
In the present invention, the concentration of hydrogen in the back shield gas described above is preferably within a range of 0.1-20%, more preferably within a range of 1% -20%, still more preferably within a range of 3%-10%, and most preferably within a range of 5%-10%.
At concentrations of less than 1%, there are cases in which there are remaining deposits. At levels of 5% or more, the deposits completely disappear. When the concentration is in excess of 20%, the effects are saturated. Accordingly, the concentration is preferably 20% or less for the purposes of economy.
Use of a noble gas, particularly argon gas, is preferable as the back shield gas. It is possible to employ other gases (for example, helium gas).
The flow rate of the back shield gas described above is preferably 6 L/min or more, and the upper limit thereof is preferably 10 L/min. By setting the flow rate to 6 L/min or more, the metal gases (metal fumes) generated during welding are no longer deposited on the inner surfaces of the welded members or the pipes, and it is possible to effectively prevent the contamination of the interior of the piping. However, if the flow rate is in excess of 10 L/min, the effects of preventing fume deposition are saturated, so that the flow rate is preferably within a range of 6 L/min to 10 L/min.
In the welding method of the present invention, when the removal of the fluoride passivated film is conducted prior to welding, it is preferable that the complete removal of the fluoride passivated film be conducted; however, even if approximately 10 nm thereof remains, it is possible to suppress the generation of deposits and the occurrence of surface roughness, so that removal may be conducted to a level of 10 nm or less.
The welding bead width during welding is preferably 4 mm or less, and a width of 2 mm or less is more preferable, and such bead widths are preferably adopted in the welding method of the present invention.
It is necessary to prevent the occurrence of surface roughness and the generation of deposits at those parts affected by heat, as well, so that it is preferable that the area from which the fluoride passivated film is removed be 5 mm or more from the butt end surfaces. In removing the fluoride passivated film, it is possible to employ a method in which the predetermined range from the butt end surfaces of the members to be welded is immersed in, for example, hot water, or to employ a method in which immersion is conducted in an aqueous solution containing hydrofluoric acid and hydrogen peroxide.
At this time, it is preferable that the temperature of the aqueous solution be within a range of 60-90xc2x0 C., and a range of 80-90xc2x0 C. is more preferable. At temperatures less than 60xc2x0 C., the removal requires a large amount of time, while if the temperature is in excess of 90xc2x0 C., it becomes difficult to control the amount removed. Furthermore, there are cases in which the surface becomes rough after removal. Since the welded part becomes molten as a result of the welding, this roughness has no effect; however, the roughness of the part affected by the heat is, of course, a source of particle and dust generation.
With respect to the period of immersion in the aqueous solution, a period of 5 minutes or more is preferable, and the upper limit is preferably 10 minutes. This is dependent on the temperature of the aqueous solution; however, if the period is in excess of 10 minutes, the fluoride passivated film may be completely removed, and once complete removal has occurred, the surface of the stainless steel which comprises the base metal is exposed, and an oxide film may be formed on the surface of the base metal. When fluoride passivation retreatment is conducted, at the parts affected by heat, a fluoride passivated film is formed on the oxide film, and there are cases in which a fluoride passivated film which is essentially in accordance with stoichiometric ratios is not formed.
Accordingly, when the removal of the fluoride passivated film by means of immersion in an aqueous solution is conducted, it is preferable that the fluoride passivated film remain in a thickness of approximately 10 nm.
It is preferable that the fluoride passivated film, which is formed on the members to be welded prior to welding, be formed as described below. The surface is first rendered in a mirrored state by means of conducting electropolishing or composite electropolishing of the surface, and then, in order to remove the moisture deposited on or adsorbed to the surface, heating (baking) is conducted for a period of approximately 10 hours at a temperature of approximately 250xc2x0 C. in a 100% nitrogen gas atmosphere (having a moisture concentration of 10 ppb or less, and more preferably 10 ppt or less). After backing, heating is conducted for a period of approximately 3 hours at a temperature of approximately 150xc2x0 C. in a gas containing approximately 1% fluorine (fluorine diluted with nitrogen). After this, heating is conducted for a period of approximately 10 hours at a temperature of approximately 250xc2x0 C. in a 100% nitrogen gas atmosphere identical to that described above. By means of such treatment, the fluorine deposited on the surface is either completely bonded to the steel or is removed, and it possible to form a fluoride passivated film essentially in accordance with stoichiometric ratios. Such a fluoride passivated film exhibits superior resistance to fluoride system gases (for example, HF gas, and WF6 gas).
Furthermore, the fluoride passivation retreatment which is conducted after welding may be conducted in a manner similar to that of the fluoride passivation treatment. However, in the case of the fluoride passivation retreatment, in order to avoid the effects of heat on the fluoride passivated film formed, it is preferable that only the welded part, or the welded part and the part affected by heat during wielding, be heated.
In accordance with the present invention, a welding method is provided for members to be welded such as pipes and the like which are employed in piping systems in, for example, semiconductor manufacturing lines, in which, in the welding of pipes and members to be welded which are subjected to fluoride passivation treatment, there is no inner surface roughness or deposits, fluoride passivation retreatment is conducted after welding, and there is superior resistance to fluorine system gases.